Piston unit having a captive spring

ABSTRACT

The invention relates to a piston unit having a captive spring for a cylinder, particularly for a tandem master cylinder of a motor vehicle, wherein the spring, with a first end, is supported at least indirectly on a piston and, with a second end, is supported on a retaining device that can be displaced relative to the piston, and the movement of the retaining device relative to the piston is limited by securing means. The invention is characterized in that the securing means have a projection, which is mounted in a non-detachable manner on the piston and whose free end is provided with a stop that limits the movement of the retaining device relative to the piston.

TECHNICAL FIELD

The present invention relates to a piston unit having a captive springfor a cylinder, in particular a tandem master cylinder of a motorvehicle, wherein the spring, with a first end, is supported at leastindirectly on a piston and, with a second end, is supported on aretaining device that is displaceable relative to the piston and themovement of which relative to the piston is limited by securing means.

BACKGROUND OF THE INVENTION

A piston unit of this type is e.g. shown in DE 33 12 192 A1 describing amaster cylinder for a brake system of a motor vehicle. The mastercylinder explained in the publication is a cylinder having two pistonsarranged one behind the other, by which two separate brake systems areactuated by means of brake pressure.

It is necessary in a master cylinder (tandem master cylinder) providedwith two pistons that the two pistons shut off, if possiblesimultaneously, two pressure chambers limited by the pistons in orderthat pressure will simultaneously develop in the two brake circuits.This necessitates that the two pistons have a defined distance from eachother in an initial position. Said distance is defined by a captivatedspring of the piston (primary piston) actuated on the inlet side. Thespring of the primary piston is relatively stiff compared to a springreturning the second piston (secondary piston) so that the second springwill yield at the commencement of the movement of the two pistons and,thus, the two pistons at a predetermined distance will simultaneouslyclose the openings that lead to a reservoir. In order to adjust thelength of the first spring at the primary piston, which is defined bythe position of the captivation, the above-mentioned patent applicationarranges for a screw having a screw head, which forms a stop for asleeve that is movable with respect to the screw. Said sleeve, with afree end, supports a free end of the stiff spring. Another end of thespring bears against the primary piston. Readjustment of the screwrenders it possible in the prior art piston unit to adjust the length ofthe spring. It is, however, disadvantageous, in the piston unit knownfrom the art that both the screw and the piston must have a thread andthat the screw can twist in relation to the piston.

BRIEF SUMMARY OF THE INVENTION

An object of the invention is to simplify the design of the piston unitand additionally improve the reliability of the mode of operation of thepiston unit.

This object is achieved in that the securing means include a projectionthat is non-detachably arranged at the piston and whose free end isprovided with a stop that limits the movement of the retaining devicerelative to the piston. Consequently, the invention principally residesin a non-detachable arrangement of a projection at the piston. Thisarrangement reliably prevents a change in position of the piston.

According to the invention, the projection at the piston can be providedby means of a metal forming process, and it can be welded, cemented,wedged by frictional engagement, or riveted to the piston. Thus, theinvention covers all coupling techniques connecting the projection tothe piston in a simple and non-detachable manner.

The invention can also be implemented in a master cylinder having onlyone single piston. Likewise, it is not absolutely necessary that thecylinder is used for a brake system.

A solution being especially favorable for manufacturing the projectionis achieved in that the projection is formed integrally with the piston,preferably by extrusion. The projection of the invention is hence shapedout of the material of the piston, with the result that an additionaloperation for coupling the projection with the piston is deleted. Tothis end, an appropriate workpiece is inserted into an appropriatelyconfigured mold, and the piston including the projection is shaped bymeans of a die. As this occurs, the material for producing theprojection flows into a corresponding opening of the die. However, theprojection can also be formed by way of turning the piston material.

As has been explained hereinabove, it is necessary to provide a stopthat defines the maximum expansion of the spring in its longitudinaldirection. To this end, the position of the stop must be fixed veryprecisely so that the tandem master cylinder operates properly. Thus, itis preferred that the stop is fastened to the piston in a form-fit.

To be able to keep very close tolerances, the projection is preferablycylindrical and includes a step for abutment of a disc at least in thearea of the free end.

In a favorable improvement of the invention, the annular disc riveted tothe projection forms the stop. The step defines the position of the discin this arrangement. The annular disc can be mounted on the step, and ashoulder of the projection extends through an opening in the annulardisc and can be deformed by a suitable metal forming operation,preferably by wobble-riveting, thereby connecting the disc with theprojection in a non-detachable manner. It is, however, also possiblewithin the limits of the invention to connect the disc to the projectionin a non-detachable manner by way of welding or riveting.

Preferably, the stop can be formed by way of enlarging the end portionof the projection by upsetting. A reduction of the components of thepiston-and-cylinder unit is achieved thereby.

In a favorable improvement of the invention, the piston has a bottom anda bowl projecting from it that accommodates the spring and theprojection at least in part. There is a free space between an outsidewall of the die and an inside wall of the mold, into which materialflows, to arrange a mold shaped like this. Aluminum has proved to be asuitable material for the piston. However, other deformable materialssuch as steel, brass, or plastics can also be used. This design of thepiston allows reducing the mounting space.

The assembly of the piston in the cylinder is simplified because thepiston is shaped in an extrusion process and includes on one outsidesurface recesses used to accommodate a push rod that moves the piston.

BRIEF DESCRIPTION OF THE DRAWING

FIG. 1 is a cross-sectional view of a piston of a piston unit of thepresent invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

The drawing shows a cross-sectional view of piston 1. Piston 1 includesa bottom 10 and a bowl 2 projecting from the bottom. A cylindricalprojection 3 extends in bowl 2 along a center line M of piston 1.Projection 3 and bowl 2 are extruded from the material of piston 1.

Step 18 is shaped at a free end 16 of projection 3 and used as anabutment for a stop 4 configured as an annular disc. A shoulder 11 ofprojection 3, shown in dotted lines, projects through an opening in theannular disc 4 and can be shaped to become a rivet head 5 e.g. by meansof wobble-riveting, with the result that disc 4 is non-detachablyconnected to projection 3.

Shoulder 11 and step 18 can be produced by means of turning of theprojection 3. Step 18 allows very precisely defining the position ofdisc 4 and maintaining narrow tolerances.

A flange 6 of a retaining device 8, pointing radially inwards and beingconfigured as a sleeve, makes catch at a side of disc 4 facing piston 1.At a free end, sleeve 8 includes a circumferential flange 7 pointing ina radially outward direction. A spring 9 is supported with a first end14 on the bottom 10 of piston 1 and with a second end 15 on the flange 7of sleeve 8 and is thus biased. The maximum expansion of spring 9 in thelongitudinal direction is defined by sleeve 8. Disc 4 limits themovement of sleeve 8 with respect to piston 1.

It is possible to fix the position of disc 4 by means of upsetting thefree end 16 of projection 3. Finally, it is possible under certaincircumstances to manage without disc 4 by way of forming a stop 4 forsleeve 8 by upsetting the free end 16, with the projection 3 being upsetuntil stop 4 has adopted the proper position.

Recesses 13 used to accommodate a push rod that moves piston 1 areprovided on an outside surface 19 of piston 1.

1-9. (canceled)
 10. Piston unit having a captive spring for a cylinder,in particular a tandem master cylinder of a motor vehicle, wherein thespring, with a first end, is supported at least indirectly on a pistonand, with a second end, is supported on a retaining device that isdisplaceable relative to the piston and the movement of which relativeto the piston is limited by securing means, wherein the securing meansinclude a projection that is non-detachably arranged at the piston andwhose free end is provided with a stop that limits the movement of theretaining device relative to the piston.
 11. Piston unit as claimed inclaim 10, wherein the projection is formed integrally with the piston byway of extrusion.
 12. Piston unit as claimed in claim 11, wherein thestop is fastened to the piston in a form-fit.
 13. Piston unit as claimedin claim 12, wherein the projection is cylindrical and includes a stepfor abutment of a disc at least in the area of the free end.
 14. Pistonunit as claimed in claim 13, wherein the annular disc riveted to theprojection forms the stop.
 15. Piston unit as claimed in claim 14,wherein the stop is fastened to the piston by way of wobble-riveting.16. Piston unit as claimed in claim 11, wherein the stop is provided bydeforming the free end of the projection.
 17. Piston unit as claimed inclaim 15, wherein the piston includes a bottom and a bowl projectingfrom it, said bowl accommodating the spring and the projection at leastin part.
 18. Piston unit as claimed in claim 15, wherein the piston isformed in an extrusion process and, on an outside surface, includesrecesses used to accommodate a push rod that moves the piston.